Imprints of Catastrophic Cyclones in Trace Element Records (Mg, Na, S, P) of Tropical Stalagmites

Whereas trace element variations in speleothems have been widely used to solve problems concerning groundwater residence times, growth rates, rainfall and air temperature histories, their application as proxies of severe storm events has only recently been considered. Here we test whether trace element proxies in stalagmites from Niue, (19° 00'S; 169° 50'W), one of the largest carbonate island in the South Pacific, record passing severe cyclones. Niue Island offers an unparalleled opportunity to carry on the test because (i) it lies within the belt of the South Pacific cyclones that strike the island periodically (ii) it supports flank-margin caves that are strung on the leeward site along a narrow terraced strip rising to a height of 20 to 23 m above sea level and terminating at the coast in steep sea cliffs, and (iii) the caves contain actively growing speleothems overlapping in time with the instrumentally recorded cyclones that hit the island. Niue Island is in the path of the SE trade winds that carry sea spray to the windward site while the leeward site is sheltered. In contrast, the leeward island is battered by rainfall deluges mixed with sea spray and storm surges only during severe cyclone events. Two actively growing stalagmites whose chronology has been previously established (Rabsury, M. and P. Aharon, G3, 2006) have been investigated using high-resolution trace element profiling and X-ray mapping by electron microprobe. The stalagmites, sampled in 2002, contain almost two century-long records and consist of sub-annual couplets alternating between thick, light calcite bands deposited during the austral monsoon summer and thin, dark calcite layers deposited during the austral dry winter. Comparison of the optical couplets with elemental X-ray maps reveals that the thin dark layers are higher in Mg relative to the light thick layers, and typically Mg correlates inversely with P and S. The sub-annual cycles are disrupted by approximately seven abrupt increases in Mg, S, Na and P concentrations throughout the stalagmites. The youngest abrupt change occurred around 1989-1990 AD when two consecutive severe cyclones (Fili in 1989 and Ofa in1990) hit directly the northwest leeward coast and caused considerable damage in life and property. The observed trace element increases (Mg~1.4 wt%, S~0.3 wt%, Na~0.25 wt%, and P~0.08 wt%) are accompanied by changes in stalagmite color and layering disruptions. We will report the fit between older abrupt chemical changes and known cyclone events, and discuss the processes controlling trace element concentrations in these stalagmites. References Rasbury, M., and P. Aharon (2006), ENSO-controlled rainfall variability records archived in tropical stalagmites from the mid-ocean island of Niue, South Pacific, Geochem. Geophys. Geosyst., 7, Q07010, doi:10.1029/2005GC001232.